Memoized safe for objects

Making `_.memoize` safe for arbitrary objects.

Previously we ran into issues when running something similar to:

```js
var memoized = _.memoize(function (person) {
  return 'hello ' + person.name;
});

var first = memoized({ name: 'Jake' });
// hello Jake

var second = memoized({ name: 'Jon' });
// hello Jake
```

Upon inspection of the `memoized.cache` `Object` we found that the keys were effectively the `.toString()` value of the key, hence `Object`s not being safe.

I have modified the cache so it is now an `Array` which has the following schema:

```js
var memoized = _.memoize(function (person) {
  return 'hello ' + person.name;
});

var first = memoized({ name: 'Jake' });
var second = memoized({ name: 'Jon' });

var cache = memoized.cache;
// [
//   { key: { name: 'Jake' }, cached: 'hello Jake' },
//   { key: { name: 'Jon' }, cached: 'hello Jon' }
// ]
```

In order to the deep comparison of arbitrary objects `_.identity` is used.